A pane is a both a container and a containee area. It serves only as a geometry management device. A pane% cannot be hidden or disabled like a panel% object.
A pane% object has a degenerate placement strategy for managing its children; it places them all in the upper left corner and does not stretch any of them. The horizontal-pane% and vertical-pane% classes provide useful geometry management.
The grow-box-spacer-pane% is intended for use as a lightweight spacer in the bottom-right corner of a frame, rather than as a container. Under MacOS, a grow-box-spacer-pane% has the same width and height as the grow box that is inset into the bottom-right corner of a frame. Under Windows and X, a grow-box-spacer-pane% has zero width and height. Unlike all other container types, a grow-box-spacer-pane% is unstretchable by default.